A tunneling machine can comprise a motorized drive and chassis for locomotion, particularly a caterpillar or track drive, an excavating or digging arm mounted pivotally on a swivel base and rotatable about a vertically directed swivel base axis passing through the swivel base, and a digging tool attached to the digging arm.
The digging arm can be lowered and raised by pivoting it about a substantially horizontal axis and, if necessary, is also rotatable about a digging or excavating arm axis passing lengthwise through the digging arm.
A ground-contacting mechanism can project from the front side of the motorized drive and behind this a conveyor can be provided for carrying the crushed stone and earth to the rear of the machine.
This apparatus commonly has a ground-contacting mechanism attached to the chassis and drive and which acts as a loading apron or mechanism which feeds the earth and stone debris to a conveyor and thereby removes the debris from the digging location.
In the known earth movers of this type the digging arm is cantilevered from the drive and chassis and has a length approximately equal to the cantilevered or forwardly projecting portion.
In excavation the digging arm can be moved with two degrees of freedom horizontally and vertically. The excavation is, however, accomplished chiefly by vertical motion of the digging arm from above to below or from below to above (or by a motion which has suitable displacement components in the vertical direction).
The employed force or at least the vertical component thereof is herein called the pressing force. This force will produce by way of a particular arrangement a reaction upon the digging arm up and down around the horizontal axis. The reaction force and the reaction moment must be absorbed by the motorized drive and chassis, which is either supported externally at the digging site or must be made up by the weight of the apparatus which must then be provided of a suitable heavy weight. The conventional belief is that the ratio of the apparatus weight to the pressing force should be at least about 5:1, if the apparatus is not to be braced externally.